Look around the NFL right now: blown-out knees, snapped tendons, broken bones, pulled muscles and beaten-up bodies. Every day it seems another player is put on the shelf for weeks or months or the season.

But when one door closes in the NFL, another opens for the next guy on the depth chart.

Maybe it is a draft pick that was expected to develop (slowly) over the course of the season or a guy who came into camp buried on the depth chart. It doesn't matter because when an opportunity presents itself for a young player, you have to go get it.

And I love that about this game.

The Bears have two players on defense in rookie linebacker Jon Bostic and second-year cornerback Isaiah Frey who will walk through that door Friday night in the exhibition opener against the Panthers.

Bostic was expected to play behind linebacker D.J. Williams in the middle of the Bears defense and contribute immediately on special teams. But a calf injury suffered by Williams in practice put the veteran in the training room and now Bostic will run with the starting defense as he gets his first taste of live NFL action.

I've watched Bostic since offseason practices at Halas Hall, talked with scouts and have been impressed with his athletic ability. The young linebacker can run, track the ball and has made the transition from the defensive scheme he played in at Florida to matching speed down the field in the Bears' Cover-2 shell.

However, can he hit, make tackles in the open field, stack and shed to get to the ballcarrier and affect the passing game versus veteran competition?

That's why they play an exhibition schedule.

Bostic won't win the starting job Friday and I don't expect him to be perfect with his footwork or assignments. The game versus the Panthers will provide some great teaching tape to study as the preseason progresses.

But coach Marc Trestman's staff needs to see that the arrow is pointing up with Bostic. They need to see a linebacker who can take the coached techniques from practice and apply them to game situations under the lights. And Bostic has an opportunity to show the staff he is on the right track to winning a starting job as a rookie.

Frey has been the star of Bourbonnais. Day after day he shows up in camp reports and he has consistently been around the football. It's exactly what you want from a young defensive back trying to stick on the roster.

But the stakes have been raised because of a season-ending hamstring injury to veteran cornerback Kelvin Hayden. That means Frey, a former sixth-round pick who didn't play a snap in the 2012 season while working on the practice squad, has a shot to play with the Bears' first-team nickel defense.

In today's passing league, that means you're a starter.

Playing nickel back is a big job in the Bears' defensive scheme. You have to show the ability to blitz off the edge in both man and zone pressures, play a "two-way-go" (inside or outside release) without the protection of the sideline and also show up in the run game as an edge defender.

That's not easy for a young corner with very little experience.

Friday will be the first big test for Frey.

There are no game plans in the first exhibition game, the playbook is dramatically reduced and the starters will play very limited minutes. If anything, these games are nothing more than an extension of practice in which coaches want to grade out young talent on technique to get a feel for where they are developmentally.

But for Bostic and Frey, Friday night is about an opportunity to showcase their abilities as pros on a defensive unit that is expected to be the strength of this team early in the season.

I'm excited to see what they can do.

Twitter @MattBowen41

Special contributor Matt Bowen, who played at Glenbard West and Iowa, spent seven seasons in the NFL as a strong safety.